Published: April 21st, 2021
Psychophysical tools measure the functionality of the taste system for both research and health assessment purposes. This paper describes a method to measure taste detection thresholds that can determine the lowest concentration of sucrose, sodium chloride, or monosodium glutamate that can be tasted by individuals as young as 6 years.
This paper describes a two-alternative, forced-choice, staircase, tracking procedure, called the Taste Detection Threshold (TDT) test, that provides a reliable measure of sweet, salty, and umami taste detection thresholds from childhood to adulthood. Advantages of the method include procedures that are identical for children and adults, thus allowing the determination of age-related and individual differences in taste perception, if any, and tasks that can be completed in a relatively short time frame, do not rely on continuous attention or require memorization, control for subjective response biases, and minimize the impact of language development. After a 1 hour fast, participants are presented with pairs of solutions; in each pair, one solution is water, and the other solution contains varying concentrations of the tastant.
Using a whole-mouth tasting method, participants taste each solution (without swallowing and with rinsing between tastings) and then point to the solution with a taste or that tastes different from water. The concentration of the stimulus in the subsequent pair increases after a single incorrect response and decreases after two consecutive correct responses. A reversal occurs when the concentration sequence changes direction. The task is deemed completed after the occurrence of four reversals, provided there are a maximum of two dilution steps between two successive reversals, and the series of reversals do not form an ascending pattern. These additional criteria ensure greater reliability in outcomes. The TDT is then calculated as the geometric mean of the concentrations of the four reversals. This method has real-world relevance as it provides information on a dimension of taste perception that is independent of hedonics, and that can change with aging and certain disease states, making it a valuable psychophysical test.
The sense of taste functions as a gatekeeper, determining in part whether an individual rejects a food or liquid or accepts it into the oral cavity. Taste psychophysics-the study of relationships between distinct chemical stimuli and the sensations and perceptions they produce-provides important information on the functioning of the taste system1. Not only are there several basic tastes (sweet, salty, bitter, sour, umami), but each taste quality can be characterized by distinct perceptual dimensions, including how sensitive individuals are in detecting the chemical stimulus or recognizing its taste, and how much they like or dislike the taste sensation.
This article describes a psychophysical method that can be used to reliably measure taste detection thresholds (i.e., the lowest concentration of a tastant that can be detected) in individuals as young as 6 years. From childhood to adulthood, detection thresholds have been used in clinical assessments of the effects of trauma or disease states2,3 and in basic research applications, to study the effects of diet, aging, development, obesity, and smoking on the taste system, as well as genotype-taste phenotype relationships4,5,6,7,8,9,10,11.
This taste detection threshold (TDT) test, which typically takes an average of 15 min per stimulus (range: 4-35 min; median: 13 min) to complete, consists of a two-alternative, forced-choice, staircase, tracking procedure that has been used to measure the lowest concentration of sucrose, sodium chloride (NaCl), or monosodium glutamate (MSG) in solution that can be detected as a taste. As outlined herein, participants are presented with pairs of solutions; in each pair, one solution is water, and the other solution contains varying concentrations of the tastant. Using a whole-mouth-tasting method, participants taste each solution (without swallowing) and then point to the solution with a taste or that tastes different from water. The concentration of the stimulus in the subsequent pair increases after a single incorrect response and decreases after two consecutive correct responses. A reversal occurs when the concentration sequence changes direction.
The task is deemed completed after the occurrence of four reversals, provided there are a maximum of two dilution steps between two successive reversals, and the series of reversals do not form an ascending pattern. These additional criteria, which were established in clinical practice by Dr. Cowart and colleagues at the Monell-Jefferson Chemosensory Clinical Research Center2, ensure greater reliability in outcomes and enhance confidence in the validity of individual measures of taste functioning. Research studies have used this method to determine taste detection thresholds for sucrose, salt, or MSG in hundreds of healthy children as young as 6 years, adolescents, and adults4,5,6,7,8,9,10,11 and have demonstrated that the majority (>~80%) of children can complete the psychophysical task4,6,7,8, highlighting the appropriateness of the method for pediatric populations.
1. General considerations
NOTE: This protocol for the TDT test describes the procedures for preparing the taste solutions and for determining taste detection thresholds for sucrose, NaCl, or MSG, using sucrose as the example. This method has been approved by the Office of Regulatory Affairs at the University of Pennsylvania. For the research studies described herein, informed consent was obtained from each adult participant or parent/legal guardian of pediatric participants. Informed assent was obtained from each child aged seven years or older prior to participation.
2. Materials and recipes to make taste stimulus solutions
NOTE: Detailed instructions for making the stock solution (1000 mmol/L; hereafter referred to as stock) and the 16 serial dilutions of the stock solution (in quarter-log steps) for sucrose, NaCl, or MSG are provided here. Table 1 lists the concentrations of each dilution step. Figure 1 illustrates the steps to make stock solution through dilution steps 1-16. The volume of solution made will be sufficient to determine thresholds for at least four participants.
|(1/4 log units apart)
Table 1: Concentration steps and corresponding molarity of sucrose, sodium chloride (NaCl), and monosodium glutamate (MSG) solutions needed for Taste Detection Threshold (TDT) testing.
Figure 1: Step-by-step instructions to make stock solutions through dilution steps #1-16. Please click here to view a larger version of this figure.
3. The psychophysical method: TDT
Figure 2: Threshold tracking grid. (A) Recording taste detection thresholds. (B) Setup of one tray. Please click here to view a larger version of this figure.
4. Preparation of materials prior to testing
5. Preparation of participants for testing
Figure 3: Child participating in a taste threshold detection test. A pair of solutions is placed on the table in front on the participant in the order that it should be tasted. The participant is asked to taste the solution in position 1 for 5 s, to expectorate, to rinse her mouth with dH2O, and to repeat for the solution in position 2. After tasting both solutions, the participant is asked to point to the solution that has a taste or tastes different than water. Please click here to view a larger version of this figure.
6. Verbal instructions to participants
7. Investigator instructions: Taste detection thresholds
Figure 4 illustrates the tracking grid results from four representative participants (A-D). Reversals, which are changes in the direction of the participant's responses, are denoted by circles and numbered in order of occurrence to illustrate when the criteria are met. Reversals are color-coded to illustrate when the change in direction goes from incorrect to correct (green) or from correct to incorrect (red).
Figure 4A shows the tracking grid from a participant whose responses met the criteria within the first four reversals. In order of occurrence, reversals for this participant occurred at steps 8, 9, 8, and 10. This sequence met the criteria because (a) there were no more than two steps between any two successive reversals (step 8 vs 9, 9 vs 8, 8 vs 10), and (b) there were two sets of pairs in which the participant correctly identified the T twice at the same step (8). The detection threshold for this participant is determined by the geometric mean of the concentrations of those four reversals:
Geometric mean = 0.0065 M
Figure 4B shows the tracking grid from a participant with a relatively high sucrose detection threshold (low sensitivity) whose responses in the first four reversals did not meet the criteria. In order of occurrence, the first four reversals occurred at steps 9, 10, 8, and 9. Although these reversals were within two steps of each other (9 vs 10, 10 vs 8, 8 vs 9), there were not two sets of pairs in which the participant correctly identified the T twice at the same step (8 vs 9). These reversals formed an ascending pattern; therefore, criteria were not met and testing continued. Reversals 6-9 met the criteria because there were (a) no more than two steps between any two successive reversals (step 8 vs 6, 6 vs 7, 7 vs 6), and (b) two sets of two correct answers in a row were obtained at the same step (step 6). The detection threshold for this participant is determined by the geometric mean of the concentrations of those four reversals:
Geometric mean = 0.021 M
Figure 4C shows the tracking grid from a participant with a relatively low sucrose detection threshold (high sensitivity) whose responses in the first four reversals did not meet the criteria. Reversals occurred at steps 9, 10, 9, and 13. Although in two pairs (pairs 3-4 and 7-8), the participant correctly identified the tastant twice at the same step (step 9), there were more than two steps between reversals 3 and 4 (step 9 vs 13). Thus, testing continued. The last four reversals (steps 13, 12, 13, 12) met the criteria because (a) there were no more than two steps between any two successive reversals (13 vs 12), and (b) the participant correctly identified the same concentration (step 12) when given pairs 17-18 and 20-21. The detection threshold for this participant is determined by the geometric mean of the concentrations of those four reversals:
Geometric mean = 0.00075 M
Figure 4D shows the tracking grid from a participant with a relatively high sucrose detection threshold (low sensitivity) whose responses met the criteria within the first four reversals (steps 6, 7, 5, 8). There were no more than two steps between any two successive reversals (6 vs 7, 7 vs 5, 5 vs 8), and the participant correctly identified the same concentration (step 6) when given pairs 7-8 and 13-14. The detection threshold for this participant is determined by the geometric mean of the concentrations of those four reversals:
Geometric mean = 0.024 M
Figure 4: Tracking grids. (A-D) Representative data from four subjects. Please click here to view a larger version of this figure.
The TDT test is a two-alternative, forced-choice, staircase procedure that uses strict rules to meet criteria than prior methods12, thus ensuring a more stable outcome measure. Using criteria established at the Monell-Jefferson Chemosensory Clinical Research Center2, the TDT is a reliable swish-and-spit method that measures the lowest concentration of sucrose, NaCl, or MSG in solution that can be detected by taste among individuals as young as 6 years. If completed as described, including enforcing participants rinsing their mouths before and after each tasting, the results are reliable and quick and provide insight into an important dimension of taste that is independent of hedonics8.
Although the application of psychophysical tools to measure this dimension of taste is well established in the field, many methods have not been validated for use in children14. There are several critical steps in the protocol, some of which apply particularly to children [see also reference15]. First, criteria for attaining threshold should not rely solely on the occurrence of any four reversals or vary due to the age of the participant. Rather, there should be a maximum of two dilution steps between two successive reversals, and the series of reversals should not form an ascending pattern, which may be the case when the participant is simply guessing or not attending to the task. These additional criteria, which were established based on clinical experience2, allow for the evaluation of the functioning of the taste system of the individual, in part because they control for false positives, especially when the participant is simply guessing16.
Second, the procedure is forced-choice, so if participants respond that "neither" or "both" solutions have a taste, that answer is not accepted. Rather, they are told to "guess." During TDT, participants often feel like they are guessing, but that should not be accepted as evidence that they are completely unaware of the taste stimuli17. Moreover, individuals may vary in their internal criteria for what constitutes a taste sensation and hence, their willingness to say that a solution does or does not have a taste. Third, because the recency of eating affects taste perception18, standardizing the time since the participant last ate or drank anything but water is important to reduce intersubject variability caused by sensory adaptation or enhancement. Fourth, the tastants used herein are palatable and presented in solution, not in a food matrix. When a food matrix is used, longer interstimulus intervals might be required for foods to clear the palate. While this method has been used to measure detection thresholds for sour or bitter tastants among adults2,11, its use to measure detection thresholds for unpalatable tastants among some young children may be problematic due to their heightened sensitivity to some bitter tastants and their potential unwillingness to continue participation19.
A forced-choice procedure of presenting up to four pairs of ascending concentrations of bitter-tasting solutions and dH2O has been successful for pediatric populations19,20. Fifth, embedded in the context of a game, the method is sensitive to the cognitive and language limitations of children, and requires only that the participant point to the cup that contains the taste. In a recent study, 80% of the children provided sustained attention for, on average, 15 min and reached criteria8. Such information on completion of the tasks should be reported, particularly when pediatric populations are studied.
The present method has real-world relevance and has been used for assessing detection thresholds for the other basic tastes of sour (citric acid) and bitter (quinine)2 and in adults of varying ages8. Because the method does not require verbal responses, the instructions should easily be translated to other languages21, making it a valuable psychophysical tool for scientists worldwide. However, like any other psychophysical methods, there will likely be limitations in its use, particularly with younger children. The procedure may be more difficult to attain criteria for children than for adults. In one study, 20% of children did not reach criteria, compared to 5% of adults8. Reasons for non-completion included unfocused behavior, failure to understand the task, or becoming fatigued and unable to continue.
Findings from studies that used this taste TDT have contributed extensively to the diagnosis of taste ageusia in the clinic and have furthered the understanding of how taste sensitivity changes with age and health status. Clinical evaluation of patients revealed that sucrose detection thresholds ≥ 0.025 M for both sexes and NaCl detection thresholds ≥ 0.012 M for men or ≥ 0.010 M for women are considered abnormal2. Among adults, there is a gradual decline in taste sensitivity for sweet, salty, sour, and bitter tastes that continues into the eighth decade22. Younger adults typically have lower taste detection thresholds (are more sensitive) than are older adults22,23,24,25. However, children and adolescents have taste thresholds for sucrose that are higher (less sensitive)8 and that are lower (more sensitive) than those of adults for the bitter taste of propylthiouracil, with the adult pattern emerging during adolescence19,26.
Taste detection thresholds have been shown to be related to indicators of health. For example, salt taste detection thresholds positively correlated with systolic blood pressure among children who were normal weight7, whereas children with central obesity had lower detection thresholds for sucrose (more sensitive) than those without central obesity4, with similar findings among adolescents27. However, the relationship between obesity and sucrose detection thresholds was not observed in adult women, and adult women with obesity had higher detection thresholds (were less sensitive) to the savory taste of MSG9.
While research on the differences in detection thresholds between children and adults are limited, it is known that sucrose taste detection thresholds do not predict sweet taste preferences or suprathreshold intensity ratings from childhood to adulthood8,28,29, providing further evidence that taste sensitivity represents a distinct dimension of taste that is independent of preferences and thus suggesting different underlying mechanisms. Greater understanding of the complex interplay among age, dietary habits, health status, and the sensitivity of the taste system, and whether such interactions differs among the primary tastants, is an important area for future research.
The authors declare they have no competing financial interests.
Dr. Joseph is supported by National Institute of Alcohol Abuse and Alcoholism (Z01AA000135) and National Institute of Nursing Research (NINR) (1ZNR0000035-01) and NIH Distinguished Scholar funds; Dr. Mennella is supported by the National Institutes of Deafness and Other Communication Disorders (NIDCD) grants DC016616 and DC011287; Dr. Cowart's effort in refining the TDT test was supported by NIDCD grant P50 DC000214; and Dr. Pepino is supported by American Diabetes Association (ADA) grant 1-19-ICTS-092 and by the USDA National Institute of Food and Agriculture (NIFA) Hatch Project 698-921. The content is solely the responsibility of the authors and does not necessarily represent the official views of NIH, NINR, NIDCD, ADA, or USDA NIFA. The funding agencies had no role in the design and conduct of the study; in the collection, analysis, and interpretation of the data; or in the preparation or contents of the manuscript.
|Glass beaker, 2000 mL
|Glass bottles with lids, 120 mL (25)
|Glass bottles with lids, 950 mL (17)
|Graduated glass cylinders, 100 mL
|Graduated glass cylinders, 1000 mL
|Graduated glass cylinders, 50 mL
|Graduated glass cylinders, 500 mL
|Mini Cupcake, 48-cup Muffin pan (2)
|Monosodium glutamate (MSG)
|Pipets 50 mL
|Sodium chloride (NaCl)
|Sucrose, Crystal, NF
|Spectrum Chemical MFG Corp
|Volumetric flask, 2000 mL, with stopper
|Volumetric flasks, 1000 mL, with stoppers (4)
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